The present invention is generally directed to a fluid pressure sensing unit and more particularly to a variable capacitance dual pressure sensing unit which senses independently the pressure levels of a pair of fluids.
It is often desirable to sense the pressure level of a fluid by developing an electrical signal having a magnitude related to the fluid pressure. One method of accomplishing this end is to provide a sensing unit which comprises a variable capacitor wherein the capacitance of the sensing unit varies as a function of applied pressure. Suitable circuitry, such as that disclosed in copending patent application Ser. No. 793,614, filed on May 4, 1977, in the names of Allen A. Bukosky and Paul P. Monroe and which is assigned to the assignee of the present invention, transform the variable capacitance to an output voltage which has a magnitude related to the applied pressure. The capacitance-to-voltage transformation circuit disclosed and claimed in the aforementioned copending application acts upon the variable capacitance to provide an output voltage which has a magnitude linearly related to the applied pressure. As a result, the applied pressure can be easily determined.
Sensing units of the kind referred to above are well-known in the art. For example, reference may be made to U.S. Pat. No. 3,808,480 which issued on Apr. 30, 1974 and U.S. Pat. No. 3,880,009 which issued on Apr. 29, 1975 to the assignee of the present invention for descriptions of the basic sensing unit structure and operation. In the latter of these patents it is explained that the basic sensing unit takes the form of a capacitor unit which includes an electrode deposited onto a ceramic substrate forming one capacitor plate which is fixed and a flexible diaphragm sealed to the substrate in juxtaposition to the fixed plate forming the other capacitor plate which is deflectable. With the side of the diaphragm opposite the fixed plate exposed to a fluid pressure, the diaphragm will deflect toward or away from the fixed plate responsive to the applied pressure with consequent changes in capacitance between the plates.
Pat. No. 3,808,480 describes a sensing unit which includes a pair of fixed electrodes and a pair of diaphragms arranged in the above referred to relation on opposite sides of the substrate. The two fixed plates are electrically coupled together forming a second connection. The capacitance related to the pressure applied to both diaphragms is taken across the first and second connections.
Sensing units of the type just referred to have been found to be very reliable for sensing a single fluid pressure. Where it is desired to sense the pressure levels of a pair of fluids independently or the differential between the pressure levels of two fluids, it has been necessary to employ two of these sensing units. Differential fluid pressure sensing in this manner is contemplated for example in copending application Ser. No. 776,524, filed on Mar. 11, 1977, and which is assigned to the assignee of the instant invention.
While differential fluid pressure sensing using two sensor units has been found to be acceptable, it exhibits economic shortcomings by virtue of the fact that two sensor units are required. Also, its use is limited to those applications affording sufficient space to accept two sensor units. Lastly, because no two sensor units can ever be exactly alike due to variations in assembly conditions even within tight manufacturing tolerances, there will always be at least some tracking error between the two units.
It is therefore a general object of the present invention to provide a fluid pressure sensing unit which is capable of independently detecting the pressure levels of a pair of fluids.
It is another object of the present invention to provide a dual pressure sensing unit which may be utilized for determining the absolute pressures of two fluids or the pressure differential between a pair of fluids.
It is a further object of the present invention to provide a dual pressure sensing unit which minimizes pressure tracking errors during the pressure sensing of two fluids.
It is a still further object of the present invention to provide a dual pressure sensing unit which comprises fewer component parts than heretofore possible by the prior art for independently sensing the pressure levels of two fluids with consequent economy.